Industrial plants that handle volatile organic compounds (VOCs) typically experience unwanted emissions of those compounds into the atmosphere from point sources such as smokestacks and non-point sources such as valves, pumps, and vessels containing the VOCs. Emissions from non-point sources typically occur due to leakage of the VOCs from joints and seals and are referred to as “fugitive emissions”. Fugitive emissions from control valves typically occur as leakage through the packing set around the valve stem. Control valves used in demanding service conditions involving large temperature fluctuations and frequent movements of the valve stem commonly suffer accelerated deterioration of the valve stem packing set.
The United States Environmental Protection Agency (EPA) has promulgated regulations specifying maximum permitted leakage of certain hazardous air pollutants from control valves (e.g. Benzene, Toluene, 1,1,1-Trichloroethane). The regulations require facility operators to perform periodic surveys of the emissions from all control valves and pump seals. The survey interval frequency may be monthly, quarterly, semiannual, or annual. If the facility operator can document that a certain percentage of valves and pumps with excessive leakage are below a prescribed minimum, the required surveys become less frequent. Thus, achieving a low percentage of leaking valves reduces the number of surveys required per year, which may result in large cost savings.
By installing automated chemical detection systems onto devices subject to the most demanding service, leaking devices can be identified and repaired so that compliance with the EPA regulations can be more readily achieved. More importantly, installing accurate chemical detection systems provides an early warning system, one that can alert the facility operator to a potential device failure and enable preventive measures to be taken before excessive leakage occurs.
To successfully achieve the goal of deploying an automated chemical detection system in an industrial environment, the chemical detection system must contain a component that efficiently collects fugitive emissions emanating from a piece of equipment and transport the emission to a gas sensor array. This component of the chemical detection system is called the sample retrieval system. The sample retrieval system must deliver the sample stream at a known flow rate in order to permit the gas sensors to make accurate and consistent measurements of the concentration of the fugitive emission.
Employing gas sensors in an industrial environment requires designing sensors that perform satisfactorily in the presence of high relative humidity (up to 85%) through a broad temperature range (from −10 C. to +50 Celsius). The sensors must be able to discriminate between the emissions of interest and other environmental contaminants, while retaining sufficient sensitivity to detect low concentrations of the fugitive emissions. The sensors must also be able to operate in other harsh environments including areas subject to spray washing and high vibration.
Consequently, the design of a field deployed chemical detection system requires both a unique physical design and the ability to self-diagnosis fault conditions to ensure proper operation prior to reporting a leak. Numerous fault conditions may result in erroneous readings. For example, variations in flow can change the thermodynamics of chemical sensing and induce errors. Permanent shifts in sensor baseline frequency can result from undesired chemical exposures, particulate accumulation, and temperature and humidity extremes. Furthermore, certain species of chemical sensors will suffer irreversible change when exposure levels exceed their saturation limit. These fault conditions can produce measurement errors or “false positive” leak reports. Responding to false positive leak reports could be as costly as performing the manual surveys.
Numerous diagnostic methods have been previously proposed. One such method, described in U.S. Pat. No. 6,200,443 B1, requires an external stimulus containing a surrogate emission to excite the Carbon Monoxide sensors. Based upon the expected system and sensor response to the surrogate, a fault determination is made. This method is disadvantageous due to the required storage and maintenance of surrogate compounds within the measurement system. Thus, the present invention addresses the concerns set forth above.